The melt polycarbonate process is based on continuous reaction of a dihydroxy compound and a carbonate source in a molten stage. The reaction can occur in a series of reactors where the combined effect of catalyst, temperature, vacuum, and agitation allows for monomer reaction and removal of reaction by-products to displace the reaction equilibrium and effect polymer chain growth.
One common polycarbonate made in melt polymerization reactions is derived from bisphenol A (BPA) via reaction with diphenyl carbonate (DPC). This reaction can be catalyzed by, for example, tetra methyl ammonium hydroxide (TMAOH) or tetrabutyl phosphonium acetate (TBPA), which can be added in to a monomer mixture prior to being introduced to a first polymerization unit and sodium hydroxide (NaOH), which can be added to a reactor or upstream of a reactor and after a monomer mixer.
Apart from the main polymerization reaction, there is a series of side reactions consisting of chain rearrangements of the polymer backbone that lead to branching that are often referred to as Fries rearrangement. The Fries species specifically found in bisphenol A melt polycarbonates are the ester type of structures A, B, and C.

The Fries reaction is induced by the combined effect of basic catalysts, temperature, and residence time, which may make melt-produced polycarbonates inherently branched as compared with the interfacial polycarbonates, as their manufacturing temperatures are lower. Because high branching levels in the resin can have a negative effect on the mechanical properties of the polycarbonate (for example, on impact strength), a polycondensation catalyst with improved selectivity (i.e., reduced Fries promotion) is needed.
Production of polycarbonate by the melt method may also require use of two different catalysts, one of organic nature known as the beta catalyst, usually a onium salt (e.g., an hydrocarbon onium salt, such as an alkylphosphonium salt); the other catalyst (known as alpha catalyst) of inorganic nature, e.g., an alkali metal salt.
Because dual catalyst systems can at times present certain challenges to their users, there is a need in the art for improved such systems.